The present invention relates to xerographic reproduction machines, but more particularly to means for stabilizing a toner image on a photoreceptor prior to the image being transferred to a transfer member such as a sheet of paper.
In conventional xerography, a xerographic plate or photoreceptor comprising a layer of photosensitive insulating material affixed to a conductive backing is used to support electrostatic latent images. In the xerographic process, the photosensitive surface is electrostatically charged, and the charged surface is then exposed to a light pattern of the image being reproduced to thereby discharge the surface in the areas wherein light strikes the surface. The undischarged areas of the surface thus form an electrostatic charge pattern (an electrostatic latent image) conforming to the original pattern. The latent image is then developed by contacting it with a finely divided electrostatically attractable powder referred to as "toner". Toner is held on the image areas by the electrostatic charge on the surface. Where the charge is greater, a greater amount of toner is deposited. Thus, a toner image is produced in conformity with a light image of the copy being reproduced. Generally, the toner image is then transferred to a suitable transfer member (e.g., paper), and the image is affixed thereto to form a permanent record of the original document.
In the practice of xerography, the transfer member is caused to move in synchronized contact with the photosensitive surface during the transfer operation, and an electrical potential opposite from the polarity of the toner is applied to the side of the paper remote from the photosensitive surface to electrostatically attract the toner image from the surface to the paper.
It is common to treat the toner image prior to its transfer by depositing a charge onto the image and photoreceptor and sometimes subjecting the image to light afterward. Generally, the purpose of this treatment is to enhance transferability by increasing transfer efficiencies or to suppress the transfer of background toner particles. One known arrangement which is presently used in a commercial machine uses a DC positively biased AC pre-transfer corona generating device to deposit a charge onto the toner image on a positively charged photoreceptor after which the image is exposed to light. The purpose of this arrangement is to reduce the image holding charge to minimize or eliminate hollow characters while increasing the transfer efficiency. The result of this reduced holding charge, however, is toner dispersion and hence a blurred image. U.S. Pat. No. 3,984,182 discloses a DC biased AC pre-transfer corona generating device to increase transfer efficiency and suppress the transfer of background particles.
U.S. Pat. No. 2,959,153 discloses a negative pre-transfer corona generator which deposits negative ions onto a toner image on a positively charged photoreceptor. According to the patent, the effect of this is to decrease somewhat the magnitude of the image defining positive electrical charge stored in the non-conducting layer of the photoreceptor. In addition, it is stated that this treatment corrects an unfavorable condition known generally as "selenium fatigue". Again, the purpose of this arrangement is to increase transfer efficiency, not to stabilize the toner image. U.S. Pat. No. 3,784,300 discloses the combination of a DC pre-transfer corona generating device and lamp arranged such that the exposure of the photoreceptor to the lamp is subsequent and never simultaneous with the charging step. Canadian Pat. No. 989,930 discloses the combination of a D.C. pre-transfer corona generating device and lamp where the lamp again follows the corona generating device. Both of these arrangements are again intended to improve transfer efficiency and suppress the transfer of background particles, not to stabilize the toner image. Thus, what is needed is an arrangement which does not sacrifice transfer efficiency, but one which produces a strong holding field to stabilize the toner image and thereby significantly reduce toner dispersion.